Trey Parker and Matt Stone rose to popularity as the creators of the cheerfully vulgar South Park, which has been on the air far past the point when it should have stopped being funny. The show’s continued popularity has made clear that the two of them, especially Parker, have an abiding interest in songwriting. They’ve dedicated several South Park episodes to mocking particular styles of music, and one-off songs crop up pretty regularly in others. And 18 years in, they’ve written enough music to keep several cover bands busy for years.